1. Field of the Invention
The present invention relates to a radio transmitter being used for a communication mode which uses a multicarrier signal, such as OFDM (Orthogonal Frequency Division Multiplex) scheme.
2. Description of the Prior Art
Generally, to process a signal modulated with an amplitude modulation scheme, especially a signal modulated with a multi-level modulation scheme, such as a quadrature amplitude modulation (hereinafter, referred also to as QAM) scheme, a linear amplification is required to an RF power amplifier for transmitting power to an antenna. For that reason, class A or class AB has been adopted as an operation class of the RF power amplifier.
However, with the progress of broadband communications, a communication mode using a multicarrier signal, such as an Orthogonal Frequency Division Multiplex (hereinafter, referred also to as OFDM) scheme has come to be used. However, when using a conventional class A or class AB RF power amplifier in such a communication mode, an improvement in efficiency may not be expected. In other words, in the OFDM scheme, a large amount of power is instantaneously generated completely at random due to a superposition of subcarriers. That is, it means that a ratio between an average power and an instantaneous peak power, i.e., a Peak to Average Power Ratio (hereinafter, referred also to as PAPR) is large. For this reason, a large amount of DC power needs to be always maintained so as to also amplify the instantaneous peak power linearly, which is much greater than the average power. In class A operation, power efficiency is only 50% at the maximum, and particularly in the OFDM scheme, since PAPR is high, the power efficiency will be dropped to about 10%.
Meanwhile, when a saturated type amplifier can be used, a period while a drain current and a drain voltage are simultaneously generated is reduced as short as possible, thereby making it possible to suppress power consumption. The saturated type amplifier includes a class F amplifier in which a higher harmonic is controlled so that a drain voltage waveform may be a rectangular wave, or a class E amplifier or a class D amplifier in which a load condition is optimized so that a drain voltage waveform and a drain current waveform may not be overlapped.
For example, supposing that a DC power with a current of 200 mA and Vdd of 3 V is supplied, the amount of the DC power will be 600 mW. In the saturated type amplifier composed of a transistor, the current does not flow when the transistor is in OFF-state, but only the voltage Vdd is applied, resulting in zero DC power consumption. Meanwhile, when the transistor is in ON-state, the current of 200 mA will flow, but the transistor is completely conducted, so that it is assumed that a drain-source voltage VDS may be about 0.3 V at the maximum, which is a saturation voltage. In this case, it means that the amount of the DC power of 0.3×0.2=0.06, i.e., 60 mW is consumed in the transistor. The power efficiency reaches as high as (600−60)/600=90%. Since power efficiency of the class A amplifier reaches 50% at the maximum, this effect is significant.
That is, the saturated type amplifier will allow a high power efficiency. However, since the saturated type amplifier is a nonlinear amplifier, it can not be used for a signal whose amplitude level of a modulated wave changes like a QAM signal because of a remarkable deterioration of a modulation accuracy.
An Envelope Elimination and Restoration (hereinafter, referred also to as EER) method that is known as Kahn Technique has been proposed to solve such a problem (for example, refer to patent document 1).
FIG. 5 is a block diagram showing an outline of the EER method. In a transmitter shown in FIG. 5, a signal which is generated by a modulated signal generating means 50, for example, a QAM signal, is separated into a phase component and an amplitude component by an amplitude phase separating means 51. The phase component is inputted into an quadrature modulator 52 as an quadrature signal, frequency-converted by it, and outputted to a saturated type amplifier 53. Meanwhile, the amplitude component is amplified to a desired amplitude level by an operational amplifier 55, and inputted into a DC converter 54. The DC converter 54 outputs a current required by the saturated type amplifier 53 to the saturated type amplifier 53 with the amplitude component. In the saturated type amplifier 53, the phase component inputted as an RF, and the amplitude component inputted from a power supply are multiplied to restore a QAM modulated wave.
By taking such a configuration, a highly efficient amplifier such as the saturated type amplifier can be employed although it may be non-linear, thereby making it possible achieve an improvement in efficiency.
Patent document 1: U.S. Pat. No. 6,256,482B1 (page 3 of the drawings, FIG. 6)
Generally, when the modulated signal is separated into the amplitude component and the phase component, a band thereof is spread about 5 times. For example, in the case of an OFDM signal of an IEEE802.11a standard which is a standard for wireless LAN, since a signal band of a baseband is approximately 8 MHz, it will be spread to a band of 40 MHz. However, a band of the DC converter 54, for example a switching regulator, which modulates the amplitude component, is at most 1 MHz, so that the EER method of such a signal can not be achieved by using the conventional configuration.
In order to spread the band, it is necessary to improve the speed of a switching element of the DC converter (switching regulator) 54. Whereas, since an improvement in speed of the switching element will lead to a decrease in withstand voltage thereof, it is thought that a further improvement in speed beyond this may not be made.
In addition, when a series regulator is used as the DC converter 54, a product of the amount of DC conversion (difference between a supply voltage and an amplitude component voltage) and a drain current of the RF power amplifier will be given as the power consumption. In the OFDM scheme, since an average voltage of the amplitude component is equal to or less than one half of the supply voltage, the improvement in efficiency can not be expected in this case, either.
Further, in order to amplify the amplitude component without distortion also in the operational amplifier 55, the supply voltage equal to or more than a peak amplitude component needs to be maintained, so that the OFDM scheme that has a large difference between the peak voltage and the average voltage will causes a reduction in power efficiency.